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There must be thousands of beers available worldwide and, of course, beer sales are very big business. But how do we actually make beer and why do we taste the tastes in beer that we taste? Well, joining Chris Smith to explain is Professor Charlie Bamforth. He is from the University of California at Davis, where he's a professor of beer and brewing. Charlie, are you really Professor of Beer and Brewing?

Charlie - Yes, it's a hard life but somebody has to do it!

Chris - I was going to say, can I have your job if you don't want it?

Charlie - No, I think I've got the best job in the world.

Chris - Haha. What does that actually involve?

Charlie - Well basically, what I do is outreach, research and teaching. I teach people all about the science and technology of brewing beer and we have a pilot brewery here and people can actually learn how to design and create their own beers. I have a very active research programme. I also have an outreach role, teaching people in California and beyond what a wonderful drink beer really is.

Chris - I suppose you must have a queue of students outside your door, wanting to join your course. When we actually talk about making beer what's actually involved in the process?

Charlie - Well, it starts in the barley fields, where you're producing really good quality malting barley. Then the malting barley has to be germinated in a very controlled way to soften it and to develop the enzymes that will break down the starch. Then the malt (the germinated barley) that is produced is dried and kilned for colour and flavour. That then goes to the brew house and is ground up and extracted with hot water. The starch degrading enzymes, the amylases, break down the starches into sugars. Then the liquid which is produced or extracted is boiled with hops to extract bitterness and aroma. This is then cooled and the pitched with yeast (Saccharomyces). And the yeast, of course, produces alcohol and some other flavours as well. Then there's a sort of cleaning-up process: some filtration, some stabilisation and it's packaged.

Chris - So why is some beer flat and other beers are quite fizzy?

Charlie - Well, yeast produces alcohol and carbon dioxide in the classic fermentation process. It doesn't produce all the CO2 that you would want in a fairly highly carbonated beer. For some beers some more CO2 is introduced but for other beers it's allowed to drift away. A classic cask ale from England is fairly low in carbonation, perhaps 1.1 volume or 1.2 volume - something like that. Some of the wheat beers in Germany are very highly carbonated, they've got three times more CO2 than liquid.

Chris - I've got an email here from Gregory Staradub. He's on the East Coast at MIT. He says, "I sometimes brew my own beer. When I open a bottle of beer that I've brewed, sometimes it behaves nicely and fizzes up all over the place but this doesn't seem to be correlated with how long the beer has spent in the bottle. Since I reuse the bottles I'm wondering, therefore, if some bacteria have got into the beer and they keep fermenting it in the bottle. Does this theory make sense and what other explanations might there be?"

Charlie - Well, there's all sorts of potential problems with brewing beer yourself. The key secret to brewing good beer is hygiene, hygiene, hygiene. It's always possible that there's residual sugar that's not been fermented properly in the fermenter, that's left behind in the bottle. Sometimes they may be leakers and the CO2 may come out of the cap. There are all sorts of possible explanations.

Chris - And Alexis Waldo says, "what makes Guinness® or stout so dark, thick and foamy and so good compared to the lighter, clear beers that you get elsewhere and places like the US?"

Charlie - Well, there are many excellent beers, some of them very light, some of them very dark. The colour of Guinness® is due to roasted cereal, roasted barley. They have a very intense heating process. The sugars and the amino acids and the grains are cooked together to give very, very dark colours. They give very roasted flavours. The foam: one of the main reasons why Guinness® foams so well is, apart from CO2 producing foam, they use nitrogen gas to give it extremely stable foams. The bubbles contain nitrogen gas and this is much more stable than CO2. Guinness® pioneered that technology.

Chris - Now, I'll let you off if you haven't got a clue about this one because, quite frankly, I was shocked! This is from Kay and she says, "My daughter recently went on a school trip and she was told that in Tudor times, if beers were poured with no head on them then they would put dead mice in the beer! Can you explain what this would achieve and why?"

Charlie - I've never heard that one! There's all sorts of crazy mythologies and truisms that people have handed down the ages. Possibly one of the things with mice is that some beer can get contaminated with Brettanomyces and Brett classically has a barnyard type, sometimes called a 'mouse urine', type of flavour. Maybe something's got lost in the telling.

Kat - Urrghh. So you're down the pub and you just put a bit of mouse wee in you pint. Anyway, we hear so many messages like alcohol is really bad for you and gives you cancer and gives you heart disease but in moderation are there actually health benefits to drinking beer?

Charlie - Yeah, there are. The red wine guys have made a bigger thing about this but it's just the same. The truth is the same for beer. One or two beers a day could stem the risk of atherosclerosis. It's been shown extensively, so it's actually good for you. Beer also is a rich source of silicon. There's a guy over there in the UK who's done a lot of work on beer countering things like osteoporosis. Beer is a significant source of several B-vitamins and antioxidants. Beer in moderation, I stress the moderation, is a good component of a diet.

Kat - Is the effect on atherosclerosis just from the alcohol content then?

Charlie - The alcohol: many people now believe the alcohol is the key ingredient in cutting down the risk of atherosclerosis. It's not some fancy antioxidant. People talk about resveratrol in red wine but you'd have to have a phenomenal number of bottles a day to get the amount needed.